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New I2C library for Teensy3

Hello all,

This thread details an enhanced I2C library for the Teensy 3.x, and LC devices (it does not support AVR based Teensy devices). The historical content of this post has been moved to the i2c_t3_history.txt file, which is linked below. This post details usage of the library below. If anyone has problems or requests please post to this thread.

This library is designed to operate from the Arduino/Teensyduino development system. However this is not strictly required as the files can be used independently. Recent releases of the library are bundled with the Teensyduino software available here. Follow the instructions on that page for installation.

The library can also be downloaded separately (eg. for updates), and used by unpacking the library contents into your sketchbook/libraries folder.

To use with existing Arduino sketches, simply change the #include <Wire.h> to #include <i2c_t3.h>

Example sketches can be found in the Arduino menus at: File->Examples->i2c_t3

The latest version of the library provides the following:

For Teensy 3.0, there is one I2C interface: Wire

For Teensy 3.1, 3.2, LC, there are two I2C interfaces: Wire, Wire1

For Teensy 3.5, there are three I2C interfaces: Wire, Wire1, Wire2

For Teensy 3.6, there are four I2C interfaces: Wire, Wire1, Wire2, Wire3

Some interfaces have multiple sets of pins that they can utilize. For a given interface only one set of pins can be used at a time, but for a device configured as a bus Master the pins can be changed on-the-fly when the bus is idle.

In functions that require a pin specification there are two ways to specify it. One is to use the pin enum as shown in the table below under "Pin Name". This will restrict the pin choices to the listed pin pairings. The other method is to specify the SCL, SDA pins directly (in that order), using any valid SCL or SDA pin given the device type and interface used. If pins are not given on initial setup then defaults are used as indicated below (based on device type and bus).

The mapping of device types, available pins, and interfaces is as follows. Note that these are not physical pin numbers, they refer to the Teensy pin assignments, which can be viewed here: https://www.pjrc.com/teensy/pinout.html

Note: in almost all cases SCL is the lower pin #, except cases marked *

On some devices the pins for the 2nd and higher number buses (Wire1, Wire2, Wire3) may reside on surface mount backside pads. It is recommended to use a breakout expansion board to access those, as the pads are likely not mechanically robust, with respect to soldered wires pulling on them. There are a number of breakout boards for this purpose such as these:

The I2C bus is a two-wire interface where the SDA and SCL are active pulldown and passive pullup (resistor pullup). When the bus is not communicating both line voltages should be at the high level pullup voltage.

The pullup resistor needs to be low-enough resistance to pull the line voltage up given the capacitance of the wire and the transfer speed used. For a given line capacitance, higher speed transfers will necessitate a lower resistance pullup in order to make the rising-edge rate faster. Generally the falling-edge rates are not a problem since the active pulldowns (typically NMOS) are usually quite strong. This article illustrates the effect of varying pullup resistance:http://dsscircuits.com/articles/86-a...l-up-resistors

However, if an excessively low resistance is used for the pullups then the pulldown devices may not be able to pull the line voltage low enough to be recognized as an low-level input signal. This can sometimes occur if multiple devices are connected on the bus, each with its own internal pullup. TI has a whitepaper on calculating pullup resistance here:http://www.ti.com/lit/an/slva689/slva689.pdf

In general, for a majority of simple I2C bus configurations a pullup resistance value in the range of 2k to 5k Ohms should work fine.

Teensy Pullups

Due to the situation with internal pullups, it is recommended to use external pullups for all devices in all cases (except in special cases for the 3.0/3.1/3.2 devices).

Regarding the Teensy devices, the library provides an option to use either internal pullups or external pullups (by specifiying I2C_PULLUP_INT or I2C_PULLUP_EXT on the bus configuration functions). For most cases external pullups, I2C_PULLUP_EXT, is the preferred connection simply because it is easier to configure the bus for a particular resistance value, and for a particular pullup voltage (not necessarily the same as the device voltages, more below). Note, when using external pullups all devices should be configured for external.

That said, sometimes internal pullups, I2C_PULLUP_INT, are used to simplify wiring or for simple test scenarios. When using internal pullups, generally only one device is configured for internal (typically the Master), and Slave devices are configured for external (since they rely on the Master device to pullup). It is possible to have multiple devices configured for internal on the same bus, as long as the aggregate pullup resistance does not become excessively low (the resistances will be in parallel so the aggregate will be less than the lowest value).

The internal pullup resistances of the Teensy devices are as follows:

Teensy LC - ~44k Ohms

Teensy 3.0/3.1/3.2 - ~190 Ohms (this is believed to be a HW bug)

Teensy 3.5 - ~150 Ohms (this is believed to be a HW bug)

Teensy 3.6 - ~25 Ohms (this is believed to be a HW bug)

None of these internal pullups is a particularly good value.

The Teensy 3.0/3.1/3.2 value of ~190 Ohms is very strong (it is believed to be a HW bug), however in most cases it can work fine on a short bus with a few devices. It will work at most any speed, including the max library speeds (eg. breadboard with 3.0/3.1/3.2 device and a few Slave devices usually works fine with internal pullups). That said, multiple devices configured for internal pullups on the same bus will not work well, as the line impedance will be too low. If using internal pullups make sure at most one device is internal and the rest are external.

On the other hand, the Teensy LC value of ~44k Ohms is very weak. An LC configured for internal will have trouble running at high speeds in all configurations.

The Teensy 3.6 internal pullup is essentially a short, and is unusable.

Pullup Voltages

Some consideration should be given when connecting 3.3V and 5V devices together on a common I2C bus. The bus voltage should be one or the other, and there should not be multiple pullups connecting to different voltages on a single line.

Sometimes devices supplied at 5V will communicate fine if the I2C bus is at 3.3V, because the logic high/low thresholds are biased towards ground more than supply. However if a 5V device truly requires a 5V I2C signal, whereas other devices on the bus require 3.3V signal, there is a method to accomplish this.

The library now supports arbitrary I2C clock rate frequencies, which can be specified directly, eg. 400000 for 400kHz. The I2C clock rate is set via a divide ratio from the F_BUS frequency (except for Wire1 bus on LC device which uses F_CPU). There is a fixed list of divide ratios available, and the library will choose the nearest available ratio when attempting to produce a requested I2C rate.

Note that at high speeds the specified clock is not necessarily equivalent to actual SCL clock speeds. The peripheral limits the actual SCL speeds to well below the theoretical speeds (both in terms of actual bit clock frequency and throughput rates).

To get a better idea of throughput the transfer time for a 128 byte transfer across different F_CPU/F_BUS/I2C Rate combinations has been measured on a Teensy 3.1 device. This behavior generally applies to all devices. This is shown below.

There are three modes of operation: Interrupt, DMA, and Immediate. The operating mode of the I2C can be set in the begin() or setOpMode() functions, using the opMode parameter which can have the following values:

I2C_OP_MODE_ISR - Interrupt

I2C_OP_MODE_DMA - DMA

I2C_OP_MODE_IMM - Immediate

Interrupt mode is the normal default mode (it was the only mode in library versions prior to v7). It supports both Master and Slave operation. The two other modes, DMA and Immediate, are for Master operation only.

DMA mode requires an available DMA channel to operate. In cases where DMA mode is specified, but there are no available channels, then the I2C will revert to operating in Interrupt mode.

Similarly, for Interrupt mode to work the I2C ISRs must run at a higher priority than the calling function. Where this is not the case, the library will first attempt to elevate the priority of the I2C ISR to a higher priority than the calling function. If that is not possible then it will revert to operating in Immediate mode.

Examples are divided into two categories, basic and advanced. Basic examples are demonstrate basic "Arduino-like" function of the library. Advanced examples demonstrate more complex scenarios, such as multi-bus, concurrent Master/Slave, and background transfer (ISR or DMA) operations.

basic_master - this creates a Master device which is setup to talk to the Slave device given in the basic_slave sketch.

basic_master_mux - this creates a Master device which can communicate using the Wire bus on two sets of pins, and change pins on-the-fly. This type of operation is useful when communicating with Slaves with fixed, common addresses (allowing one common-address Slave on each set of pins).

basic_master_callback - this creates a Master device which acts similar to the basic_master sketch, but it uses callbacks to handle transfer results and errors.

basic_slave - this creates a Slave device which responds to the basic_master sketch.

basic_slave_range - this creates a Slave device which will respond to a range of I2C addresses. A function exists to obtain the Rx address, therefore it can be used to make a single device act as multiple I2C Slaves.

basic_scanner - this creates a Master device which will scan the address space and report all devices which ACK. It only scans the Wire bus.

basic_interrupt - this creates a Master device which is setup to periodically read/write from a Slave device using a timer interrupt.

basic_echo - this creates a device which listens on Wire1 and then echos that incoming data out on Wire. It demonstrates non-blocking nested Wire calls (calling Wire inside Wire1 ISR).

advanced_master - this creates a Master device which is setup to talk to the Slave device given in the advanced_slave sketch. It adds a protocol layer on-top of basic I2C communication and has a series of more complex tests.

advanced_slave - this creates a Slave device which responds to the advanced_master sketch. It responds to a protocol layer on-top of basic I2C communication.

advanced_scanner - this creates a Master device which will scan the address space and report all devices which ACK. It scans all existing I2C buses.

advanced_loopback - this creates a device using one bus as a Master (Wire) and all other buses as Slaves. When all buses are wired together (loopback) it creates a closed test environment, which is particularly useful for Master/Slave development on a single device.

I2C_BUS_ENABLE n - this controls how many buses are enabled. When set as "I2C_BUS_ENABLE 1" only Wire will be active and code/ram size will be reduced. When set as "I2C_BUS_ENABLE 2" then both Wire and Wire1 will be active and code/ram usage will be increased. Specifying a higher number of buses than exists is allowed, as it will be automatically limited by what is available on the device. The default is "I2C_BUS_ENABLE 4", to enable all buses on all devices by default.

I2C_TX_BUFFER_LENGTH n
I2C_RX_BUFFER_LENGTH n - these two defines control the buffers allocated to transmit/receive functions. When dealing with Slaves which don't need large communication (eg. sensors or such), these buffers can be reduced to a smaller size. Buffers should be large enough to hold: Target Addr + Data payload. Default is: 259 bytes = 1 byte Addr + 258 byte Data, as that is what some examples use.

I2Cx_INTR_FLAG_PIN p - these defines make the specified pin high whenever the I2C interrupt occurs (I2C0 == Wire, I2C1 == Wire1, and so on). This is useful as a trigger signal when using a logic analyzer. By default they are undefined (commented out).

I2C_AUTO_RETRY - this define is used to make the library automatically call resetBus() if it has a timeout while trying to send a START. This is useful for clearing a hung Slave device from the bus. If successful it will try again to send the START, and proceed normally. If not then it will exit with a timeout. Note - this option is NOT compatible with multi-master buses. By default it is disabled.

I2C_ERROR_COUNTERS - uncomment to make the library track error counts. Error counts can be retrieved or zeroed using the getErrorCount() and zeroErrorCount() functions respectively. When included, errors will be tracked on the following (Master-mode only): Reset Bus (auto-retry only), Timeout, Addr NAK, Data NAK, Arb Lost, Bus Not Acquired, DMA Errors. By default error counts are enabled.

I2C_DISABLE_PRIORITY_CHECK - uncomment to entirely disable auto priority escalation. Normally priority escalation occurs to ensure I2C ISR operates at a higher priority than the calling function (to prevent ISR stall if the calling function blocks). Uncommenting this will disable the check and cause I2C ISR to remain at default priority. It is recommended to disable this check and manually set ISR priority levels when using complex configurations. By default priority checks are enabled (this define is commented out).

Black functions are compatible with the original Arduino Wire API. This allows existing Arduino sketches to compile without modification.

Green functions are the added enhanced functions. They utilize the advanced capabilities of the Teensy 3.0/3.1 hardware. The library provides the greatest benefit when utilizing these functions (versus the standard Wire library).

'^' indicates optional function arguments. When not specified default values will be used.

Wire.setOpMode(opMode); - this configures operating mode of the I2C as either Immediate, ISR, or DMA. By default Arduino-style begin() calls will initialize to ISR mode. This can only be called when the bus is idle (no changing mode in the middle of Tx/Rx). Note that Slave mode can only use ISR operation.
return: 1=success, 0=fail (bus busy)
parameters:

Wire.setRate(busFreq, rate); - reconfigures I2C frequency divider based on supplied bus freq and desired rate. Rate is specified as a direct frequency value in Hz. The function will accept I2C_RATE_xxxx enums, but that form is now deprecated.
return: 1=success, 0=fail (function can no longer fail, it will auto limit at min/max bounds)
parameters:

busFreq = bus frequency, typically F_BUS unless reconfigured

rate = frequency of I2C clock to use in Hz, eg. 400000 for 400kHz. Can also be specified as a I2C_RATE_xxxx enum (deprecated), refer to Clocking Section above

Wire.setDefaultTimeout(timeout); - sets the default timeout applied to all function calls which do not explicitly set a timeout. The default is initially zero (infinite wait). Note that timeouts do not currently apply to background transfers, sendTransmission() and sendRequest().
return: none
parameters:

timeout = timeout in microseconds

Wire.resetBus(); - this is used to try and reset the bus in cases of a hung Slave device (typically a Slave which is stuck outputting a low on SDA due to a lost clock). It will generate up to 9 clocks pulses on SCL in an attempt to get the Slave to release the SDA line. Once SDA is released it will restore I2C functionality.
return: none

Wire.sendTransmission(^i2c_stop); - non-blocking routine, starts transmit of Tx buffer to slave. i2c_stop parameter can be optionally specified to indicate if command should end with a STOP (I2C_STOP) or not (I2C_NOSTOP). Use done(), finish(), or onTransmitDone() callback to determine completion and status() to determine success/fail. Note that sendTransmission() does not currently support timeouts (aside from initial bus acquisition which does support it).
return: none
parameters:

^i2c_stop = I2C_NOSTOP, I2C_STOP (default STOP)

Wire.requestFrom(address, length, ^i2c_stop, ^timeout); - blocking routine with timeout, requests length bytes from Slave at address. Receive data will be placed in the Rx buffer. i2c_stop parameter can be optionally specified to indicate if command should end with a STOP (I2C_STOP) or not (I2C_NOSTOP). timeout parameter can also be optionally specified.
return: #bytes received = success, 0=fail
parameters:

address = target 7bit slave address

length = number of bytes requested

^i2c_stop = I2C_NOSTOP, I2C_STOP (default STOP)

^timeout = timeout in microseconds (default 0 = infinite wait)

Wire.sendRequest(address, length, ^i2c_stop); - non-blocking routine, starts request for length bytes from slave at address. Receive data will be placed in the Rx buffer. i2c_stop parameter can be optionally specified to indicate if command should end with a STOP (I2C_STOP) or not (I2C_NOSTOP). Use done(), finish() or onReqFromDone() callback to determine completion and status() to determine success/fail.
return: none
parameters:

These are libraries which are known to be compatible with this I2C library. They may have been possibly modified to utilize enhanced functions (higher speed, timeouts, etc), or perhaps for general compatibility. Please contact their respective authors for questions regarding their usage.

Added new priority escalation - in cases where I2C ISR is blocked by having a lower priority than calling function, the I2C will either adjust I2C ISR to a higher priority, or switch to Immediate mode as needed.

Added new operating mode control - I2C can be set to operate in ISR mode, DMA mode (Master only), or Immediate Mode (Master only)

When I ran my debug tests, I was using just the internal pullups (approx 190 ohms). I wasn't sure the Teensy3 pulldown capability would be strong enough to work with that, but in fact on the scope I was getting around 3Vpp swings (IIRC, maybe even a bit more), and the edges were pretty square. There is a cost of course, when the bus lines are low, each wire is dumping 15mA or so, but unless it gets stuck in that state it is just transient.

As far as the name I used i2c_t3 just because it is Teensy3 specific. There is no AVR code in it, so a generic name didn't seem appropriate. I'm not familiar with Arduino library conventions though, so maybe it doesn't matter.

The I2C internal pullups are messed up in this part. This was noted on the original TwoWire code also. In non-I2C mode if the pins are configured for pullups they have reasonably high impedance, but when you switch on I2C it becomes a low impedance.

The resistance amount is kind of weird, it's really too high for just the on-resistance of the I/O FET and perhaps some ESD series R. You can actually get it even lower than 190 ohms using the high drive option. But even then it's still too high for a I/O driver on-resistance. It looks to me like someone used the wrong doping on the pullup resistors for that block (eg. they used silicided poly instead of high-R poly, perhaps a mask screwup).

In any event the bus can run with low resistance pullups just fine, as long as the current drain isn't a problem and the slave pulldowns are strong enough.

So the I2C recommended pullups are from 4.7K to 10K. (10k used for the higher bit-rates) And you are only using 190 ohms pullups? Something is not quite right!

I thought smaller resistors were needed for faster pullups. See the research Nick Gammon posted near the bottom of his I2C page. I regularly use 2k2 resistors on my I2C buses based on this research. By comparison, the 10K 'square' pulses are look more like shark fins!

I have an application where I need to use both I2C buses (interfacing two I2C devices that have the same address, so I assume they can't be on the same bus). Looking at the number of static private variables in the Wire/i2c_t3 classes, I'm guessing it was written when there was only one I2C bus. Is there some way to make this work, e.g. adding a method to set the currentPins member, or maybe calling begin() before reading, or would this involve moving all the stuff that's static into instance variables?

Looking at the number of static private variables in the Wire/i2c_t3 classes, I'm guessing it was written when there was only one I2C bus. Is there some way to make this work, e.g. adding a method to set the currentPins member, or maybe calling begin() before reading, or would this involve moving all the stuff that's static into instance variables?

It's important to understand there is only one I2C block in the part. The different pin configurations are only internal muxing to the same I2C. Understanding that however, it seems possible to do this by reconfiguring the pins, but you have to be a bit clever about it. Since the I2C hardware partially knows the state of the bus (busy, not busy, Tx/Rx, etc), you should only do this while the bus is idle (not busy).

I've managed to get a test of this working. I've added a function to the code:

That checks to see if the bus is busy, and if not reconfigures the I2C output on the indicated pins with the indicated pullup method. When the switch happens the other pins get configured as inputs using the same indicated pullup method. I've also included my test sketch - i2c_multi_master_t3. You can refer to that for an example, but it's very simple, just make sure to reconfigure between completed I2C commands (not in the middle of a command).

Ah, I was afraid that the part might behave like that. But thanks a million for finding a workaround. I'll give it a whirl tonight. I'll bet it'll work for me, since the devices are just Wii Nunchucks and I'm just polling them periodically.

And if all else fails, I'm pretty sure I have a 74HC4052 in my parts box. Thanks for the suggestion, Paul.

Excellent! This is an interesting project and there are lots of Freescale Pressure Sensors so why not start another thread describing it in more detail? With the Yamaha BC3A being discontinued, people are looking for alrernatives for breath controllers.

Excellent! This is an interesting project and there are lots of Freescale Pressure Sensors so why not start another thread describing it in more detail? With the Yamaha BC3A being discontinued, people are looking for alrernatives for breath controllers.

In general, you're going to want a sensor that maxes out at about 6 kPa (kiloPascals), which is 0.87 psi. Any higher pressure and you're likely to have an anyeurism trying to make the sensor go full-scale.

This library is excellent! Given the range of frequencies the I2C bus can be configure to run at I am guessing the I2C hardware on the Teensy3 is a lot more capable of than the one on the Teensy++2 boards.

A question: Is if there is a timeout/reset feature in case of bus lockups ? With the Teensy++2 boards I have previously worked with the I2CMaster library from DSS circuits and it has such a feature. When I started wit my 1st I2C project I encountered problems and this was a god send feature that helped me get my project on its feet - albeit a little shaky - until I had the means in terms of knowledge and analyze and fix the problem.

The timeouts are a good idea. I had a recent email exchange with someone who needed something similar. It is not a difficult thing to do since the library already has background Tx/Rx capability. It did require a little cleanup to prevent hanging the ISR though. I've augmented the endTransmission(), requestFrom() and finish() functions to incorporate timeouts. To use this requires the long form of the function calls, where timeout will be specified in microseconds:

3) Non-blocking Tx/Rx followed sometime later by finish() with timeout:
Wire.finish(timeout);

To test it I wrote an example sketch. I've uploaded a "v3" revision library, see the original post (inside it refer to the i2c_timeout_t3 sketch). It runs through the methods above.

Note that timeouts are not foolproof, if you abort a READ in the middle and a slave gets stuck outputting a low on the SDA line it will hang the bus no matter what you do, since the master cannot transmit a START or STOP (since SDA is held low). Timeouts can however help diagnose a problem if you dump messages when they occur.

Regarding that here is how to check for the timeout error condition on exit. In this case I did not append to the Arduino Wire library's idiotic and inconsistent error codes. There are various methods to do this, but a simple one is to directly do a status check after the call:

Well v3 was short lived. It turns out my method of ending the communication, particularly in the middle of Rx with a slave, was a bit too crude. When receiving from a slave the ISR needs to NAK the slave to get it to reliably release the bus, but this can't be done in a single receive byte (sending a NAK occurs on the next receive byte).

I reworked the ISR code to better handle that case. I've uploaded a v4 version, attached to the top post. If anyone runs into bugs let me know.